Silver (Ag) is one of precious metals and is known to be usable as a metal for accessories from a long time ago. Moreover, since silver has unique characteristics such as catalyst action and antibacterial action as well as excellent conductivity and optical reflectivity, silver is a promising metal used in various industrial applications such as electrode or wiring materials, materials for reflective films, catalysts, and antibacterial materials. As a utilization form of silver used for various applications, there is a case where silver particles are dispersed or suspended in an appropriate solvent. For example, in the case of silver used for formation of electrodes or wirings of wiring boards mounted on electronic components such as semiconductor devices, silver particles are formed to have a paste form, this metal paste is applied and calcined so that it is possible to form desired electrodes or wirings.
A liquid phase reduction method is a generally known method for producing silver particles. In the method for producing silver particles according to the liquid phase reduction method, a silver compound serving as a precursor is dissolved in a solvent and a reducing agent is added to the resultant solution, thereby precipitating silver. At this time, it is general to add a compound called a protective agent in order to suppress the aggregating and coarsening of silver particles to be precipitated. Since the protective agent is bonded to the silver particles which have been precipitated by reduction and suppresses the contact between the silver particles, the protective agent prevents the aggregation of silver particles.
Regarding the method for producing silver particles according to the liquid phase reduction method, it is possible to efficiently produce silver particles by adjustment of the silver compound concentration in the solvent and the type and added amount of the reducing agent, and appropriate selection of the protective agent. However, the silver particles to be produced according to the liquid phase reduction method tend to relatively increase in a particle diameter, and the particle size distribution tends to vary depending on the concentration gradient of a reaction material in a solvent.
In this regard, as a method for producing silver particles alternative to the liquid phase reduction method, a thermal decomposition method of a silver complex is reported (Patent Document 1). This method basically uses characteristics of a thermally-decomposable silver compound such as silver oxalate (Ag2C2O4), and this method is to obtain silver particles in such a manner that a complex is formed by use of this silver compound and an organic compound serving as a protective agent and the complex is heated as a precursor. In Patent Document 1 described above, silver particles are produced by thermal decomposition in such a manner that an amine as a protective agent is added to silver oxalate to form a silver-amine complex and the silver-amine complex is heated at a predetermined temperature. This thermal decomposition method allows for production of silver fine particles with an extremely minute diameter of several nanometers to a ten and several nanometers and also to obtain silver fine particles with a relatively uniform particle diameter.